Alkyl sulfonates, alkyl ether sulfonates, higher fatty acid salts and the like have been used as surface active agents for cleansing compositions. However, milder raw materials have been desired since conventional surface active agents have had a problem in the irritation to hand skin. In recent years, amino acid-based surface active agents such as N-acyl glutamates have attracted attention as a surface active agent which is mild to hand skin and high in safety, and has good biodegradability.
However, N-acyl glutamates exhibit weak foaming and often provide slimy and greasy feelings after rinsing. In addition, although alkaline cleansing compositions containing higher fatty acid salts as a main component provide a refreshed feeling, there was a problem in the feeling upon use like a stretched feeling of the skin after use thereof.
In order to solve these problems, Japanese Patent Laid-Open No. 2-268114 (Patent Document 1) has attempted to improve sliminess and foaming by defining a neutralized salt of an N-acyl aspartic acid, but as a result of the study by the present inventors, it has been found that both a refreshed feeling thereof and improvement of a stretched feeling after use thereof are still not satisfactory. Japanese Patent Laid-Open No. 10-121091 (Patent Document 2) discloses a cleansing composition containing an N-long-chain-acyl dipeptide salt in which the dipeptide moiety is composed of acidic amino acids and an N-long-chain-acyl acidic amino acid salt, and discloses that the cleansing composition is non-irritating, has a high resistance to hard water and provides a pleasant feeling upon use. However, there is no description on a refreshed feeling, and as a result of the study by the present inventors, it has been found that the refreshed feeling is not satisfactory and foaming is also not yet satisfactory. Japanese Patent Laid-Open No. 2003-183152 (Patent Document 3) discloses a creamy skin cleansing composition containing an N-acyl acidic amino acid salt, a sulfosuccinic acid surface active agent, a higher fatty acid salt and polyethylene glycol, and discloses that the composition is excellent in foaming and the quality of the foam as well as excellent in a feeling upon use. However, it has been found that a refreshed feeling thereof and foaming are not yet satisfactory. Japanese Patent Laid-Open No. 2003-171687 (Patent Document 4) discloses a cleansing composition containing N-acylglutamic acid and a salt thereof, N-acyldiglutamic acid and a salt thereof and a free fatty acid and a salt thereof, wherein all components have a common acyl group or an alkylcarbonyl group, and discloses that the composition is excellent in low-temperature stability and cream shape retention. However, it has been found that a refreshed feeling and foaming thereof are not satisfactory and it exhibits a slimy feeling after use.
In addition, Japanese Patent Laid-Open No. 2-268113 (Patent Document 5) reports that a cleansing agent using N-acyl-aspartic acid or a salt thereof as a base material among amino acid-based surface active agents has a performance advantage over those using an N-acyl glutamates because the former provides better foaming power, more refreshed feeling and better rinsing effect than the latter. However, as a result of the study by the present inventors, it has been found that when this cleansing agent is stored at 25° C. for a long period of time, there arises a problem of performance-reduction such as reduction of foaming power, reduction of cleansing power, disappearance of a refresh feeling and increase of a stretched feeling after cleansing.
In order to solve these problems, Japanese Patent Laid-Open No. 4-364112 (Patent Document 6) discloses an aqueous liquid cleansing composition containing an N-(2-hydroxyalkyl)acidicamino acid. However, it describes nothing about a refreshed feeling, and as a result of the study by the present inventors, it has been found that the composition provides no refreshed feeling and exhibits a strong stretched feeling of skin after use. Japanese Patent Laid-Open No. 5-070794 (Patent Document 7) attempts to improve storage stability of a triethanolamine salt solution of N-acyl-aspartic acid by defining its pH. However, as a result of the study by the present inventors, it has been found that there is no effect on improvement in storage stability and the solution exhibits a strong stretched feeling of the skin after use. Japanese Patent Laid-Open No. 4-180999 (Patent Document 8) attempts to improve storage stability of a composition by defining the molar ratio of N-acyl-aspartic acid to ethanolamine. However, as a result of the study by the present inventors, it has been found that the composition exhibits poor foaming and does not exhibit a satisfactory refreshed feeling.
In the case of using an N-long-chain-acylamino acid salt, in particular sodium salt or potassium salt, insolubles have often precipitated when an aqueous solution of the N-long-chain-acylamino acid salt is stored at a low temperature. In addition, when an N-long-chain-acylamino acid salt and its aqueous solution are stored at a high temperature, the N-long-chain-acylamino acid salt and its aqueous solution have often discolored to yellow. Therefore, there have been problems in that they need extra work of redissolution or filtration when they are formulated into a final product, or the final product discolors to yellow. Thus, an N-long-chain-acyl amino acid salt and its aqueous solution excellent in low and high temperature stability have been demanded.
In order to improve low-temperature stability, for example, Japanese Patent Laid-Open No. 2001-131129 (Patent Document 9) discloses the improvement of low-temperature stability by combining an N-long-chain-acylamino acid salt with a hydrophilic substance having 12 or more carbon atoms. However, as a result of the study by the present inventors, it has been found that although improvement of low-temperature stability is significant, high-temperature stability is not satisfactory, because the composition has exhibited yellowing in the evaluation of stability thereof at 50° C. for about one month. Japanese Patent Laid-Open No. 9-78085 (Patent Document 10) attempts to improve high-temperature stability by combining a glycine derivative with a metal chelating agent and an antioxidant. However, as a result of the study by the present inventors, it has been found that insolubles precipitate at 0° C., showing unsatisfactory low-temperature stability.    Patent Document 1: Japanese Patent Laid-Open No. 2-268114    Patent Document 2: Japanese Patent Laid-Open No. 10-121091    Patent Document 3: Japanese Patent Laid-Open No. 2003-183152    Patent Document 4: Japanese Patent Laid-Open No. 2003-171687    Patent Document 5: Japanese Patent Laid-Open No. 2-268113    Patent Document 6: Japanese Patent Laid-Open No. 4-364112    Patent Document 7: Japanese Patent Laid-Open No. 5-070794    Patent Document 8: Japanese Patent Laid-Open No. 4-180999    Patent Document 9: Japanese Patent Laid-Open No. 2001-131129    Patent Document 10: Japanese Patent Laid-Open No. 9-78085